1. Technical Field
This invention relates to the technology of thermally spraying hard surface coatings and, more particularly, to coatings which contain lubricant or wear-resisting particles.
2. Discussion of the Prior Art
Thermal spraying is a well-established branch of surface coating technology which produces deposits that add a variety of characteristics and properties to the coated component. It encompasses a number of different methods of spraying which differ in the materials employed and the methods used to melt them.
Essentially, these different methods fall into four basic categories: flame spraying, electric arc spraying, plasma spraying, and detonation spraying. Although these methods differ in the fuels and forms of heating they employ, and also in the nature of the feedstock material, they all retain the basic concept of creating hot particles which are subsequently atomized and projected toward a suitably prepared substrate. Upon striking the target, these hot particles deform with considerable force to produce a lamellar structure.
Wire, as a solid feedstock, has been used only with the flame spray and electric arc spray processes. The problem with the use of solid feedstock wire is that it is difficult to form a uniform homogeneous coating if it is a composite of various constituents. For example, graphite is particularly difficult to disperse and integrate into a molten body without dissolution. Adding powdered graphite either upstream or downstream of the electric arc or flame limits the desirable distribution of the graphite and may fail to prevent ablation (i.e., oxidation or dissolution) of the graphite as it is exposed to projecting gases or molten metal.
A cored feedstock wire has been created and disclosed in copending U.S. application Ser. No. 998,074 now U.S. Pat. No. 5,364,663, commonly assigned to the assignee of this invention; additive materials are contained and consolidated in a central hollow of the wire. This wire works well with electric arc spraying to induce homogeneity and inhibit ablation. However, when such cored feedstock wire is used in certain flame spray techniques such as high-velocity oxy-fuel (HVOF), variable chunks of the wire break off and are dispersed in a uniform, improperly melted manner.
Moreover, when such surface coating technology is transferred to the art of coating internal bores of a block, such as the cylinder bores of an internal combustion engine, with a composite coating we find the adhesive strength of the coating is not optimized sufficiently. It is desirable to use techniques that avoid chemical clean-up and costs associated with wet electrolytic deposition (see "Hard Surface coatings by Electric Arc Spraying", R. C. Cobb et al, Welding and Metal Fabrication, July 1988, pp. 226-231; and U.S. Pat. No. 3,929,596).
It thus remains a problem as to how to thermally spray composite coatings into the bores of an engine block constituted of a relatively low melting metal, i.e., aluminum alloy, with greater thermal energy to achieve a highly adherent coating and yet achieve exacting homogeneity in the coating.